Sludge is the semi-liquid residual material left from wastewater treatment processes, and consists of water and suspended particle pollutants such as microorganisms, mineral matter, and other particles.
It has been a challenge for Waste Water Treatment Plants (WWTP) to treat and dispose of the increasing volumes of sludge, as the communities WWTPs serve are growing. Additionally, WWTPs adherence to restrictive environmental regulations plays a central role in pushing the need for effective and efficient sludge removal efforts.
Sludge removal efforts and the effectiveness and efficiency thereof can be greatly influenced by the effectiveness and efficiency at which water is removed from the sludge, where disposal fees are directly proportional to the weight of the sludge being removed. The weight of the water entrenched in the sludge makes up a majority of the weight of the sludge. It is therefore highly desirable to extract as much water from the sludge as possible. To extract the water from the sludge would greatly reduce sludge transport removal costs, and thereby effectively enhance sludge removal efforts by increasing its efficiency.
A method used to extract water from the sludge is the mixing of the aqueous sludge solution with a polymer used to aid the aggregation of the particles within the aqueous sludge solution, to thereby separate the particles from the water, enabling the water to be extracted from the aqueous solution more efficiently. In wastewater treatment, this method is part of the generically named “dewatering” process. Generally, following the separation of the water and particles while within the aqueous sludge solution, a mechanical means for separation (e.g. centrifuges, belt presses), which are known to persons of ordinary skill in the art of dewatering for a wastewater treatment process, is soon implemented to mechanically extract the water from the remaining solid sludge particles. The instant invention will provide an apparatus and method that will increase the efficiency of this dewatering process, by providing an apparatus and method that will increase the efficiency of the mixing of polymers into the sludge—where mixing efficiency is directly proportional to particle aggregation and water separation, and where the rate of air introduction and the mixing energy are independently variable.
In the aqueous sludge solution, many of the suspended particles have a negative surface charge and are surrounded by positive counter-ions, which cause the particles in the sludge to repel each other, in comport with the electrical forces of repulsion. In providing for an efficient dewatering process it is highly desirable for the suspended particles of this aqueous sludge stream to attract to one another and aggregate. To aid this process of particle aggregation, polymers are added to the aqueous sludge stream. Polymers are added to the aqueous sludge, and through adsorption—the adhesion of atoms, ions, or molecules from the polymers to the surface of the particle pollutants, where a film of the polymers is left on the surface of the particles—the particles are enabled to overcome the electrical forces of repulsion, and become attracted to one another, in comport with Van der Waals forces. The effectiveness of the polymer addition to the sludge side-stream, to further the process of adsorption, increases the effectiveness of wastewater treatment, where it aids particle aggregation.
Rapid mixing is the process by which the polymer is rapidly dispersed throughout the aqueous sludge solution, where the particles are brought into contact with one another to form flocs (through the process of flocculation), which enables the free water to be removed in dewatering. The rapid mixing stage is quite possibly the most important stage of the dewatering process, and drives the effectiveness and efficiency of the dewatering process as a whole. The principal parameter governing the rate of flocculation is the velocity gradient applied to the sludge/polymer mixture. Velocity gradients can be induced by multiple means, readily known by those ordinarily skilled in the art of pipe liquid flow and air/liquid contact in pipe, such as through the use of a venturi (known in function to be a constricted area of a pipe, narrowed to effect a pressure and velocity change in the fluid traveling through the narrowed section). The instant invention will induce a velocity gradient change through a mechanical means (hereinafter also described as a flow restriction device), such as a venturi, to increase mixing efficiency.
Additionally, where turbulence is created, a pressure gradient effect is also anticipated. Where the infusion of air and polymer into an aqueous sludge solution occurs and creates an increase in turbulence, then a pressure gradient change will also be exhibited, and mixing will be aided. The instant invention will aid in increasing the pressure gradients through the novel infusing of air and polymer into the area of the pipe directly downstream of venturi-induced gradient change. The said mixing zone will hereinafter be called the zone of intense mixing.
It is highly desirable to increase the efficiency of the dewatering process—intended to increase its total solids (T.S.) content. Even small dewatering efficiency improvements (1% T.S.-2% T.S.) will have a significant impact on improving overall disposal economics. Unfortunately, increases in the efficiency of dewatering occur with increases in mixing energy or increases in polymer dosing, which are accompanied by undesirable increases in operational costs. The desirableness of efficiency improvement methods, such as increasing mixing energy or polymer dosing, is thereby mitigated by their cost-prohibitive nature, which makes the necessity of an economical apparatus and method for increasing dewatering efficiency readily apparent, which is satisfied with the present method and apparatus.